Harold Hall

Workshop Projects

Making Soft Jaws for the 3 Jaw Chuck. Harold Hall

I have described elsewhere on the site the purpose for, and method of, using soft
jaws. Realising that many will not have these available and be unable to acquire
them for their aged chuck the following is a method for making your own. If you decide
to do this it would be a good idea to make a second set as being set up for one set
the second can be made in much less time.

Making standard soft jaws.

Making your own soft jaws will be a time consuming task and probably only a viable
option if you have a chuck for which jaws cannot be purchased. Having said that,
if you have a day to spare, making a set would be worthwhile for some just for the
satisfaction of making something quite different. However, using commercially available
soft jaws with my simplified design of hexagonal add-ons would also be worth considering.

The theory (mine)

I say "mine" as I have no access to any design theory and is purely as I see them.

It would be easy to fall into the trap of thinking that a scroll with mating jaw
is only a threaded component but having a flat form. Whilst this is true to a point,
for example the pitch of the scroll is constant allowing the jaw to move along it,
the radius of the scroll is of course increasing from its centre to the outer ring.
This means that the jaw teeth have to work with a changing radius as the jaw moves
in and out, what radius then are they to be made?

The above though is an oversimplification as not only is the radius changing between
one pitch and the next but also continuously and therefore across the width of the
jaw, Sk. 1 shows the effect. Because of this a compromise has to be adopted as it
is impossible for the jaw to mate precisely with the scroll at any other than one
point. Sk. 2 shows two possible approaches though I feel all will realise that approach
"b" is a non starter as wear will be much more than the method adopted in "a". For
this to be achieved at all pitches of the scroll the curvature on the jaws teeth
must be equal or greater than the curvature in the centre of the scroll where the
curvature is the greatest.

The above is though only applicable for the situation where the chuck is being used
to close down onto a workpiece. In this case, as the above discusses, the inner surface
of the scroll is acting on the outer surface of the jaws teeth. What then if the
jaws are being expanded into an outer workpiece where the outer surface of the scroll
is acting on the inner surface of the jaws teeth.